The present invention relates to a friction material. More particularly, the present invention relates to a friction material useful for automobiles, railway vehicles and various industrial machines. The friction material contains, as a principal material, an inorganic substance combined with artificial pulp particles comprising a heat-resistant aromatic polymer material.
It is known that in recent years, a significant increase in size and speed of vehicles caused a great demand for improving the function of friction articles such as brake linings, disc brake pads or clutch facings in vehicles. That is, friction articles are required to have not only a high coefficient of friction when the articles are brought into contact with corresponding sliding surfaces, but also, high stability in friction performance of the friction material irrespective of any changes in temperature, sliding speed and pressure on the sliding surfaces. Also, the friction articles should exhibit excellent wear-resistance and heat-resistance. Furthermore, it is necessary that friction articles do not make abnormal noises when contacting the sliding surfaces. Heretofore, various types of friction materials containing, as a principal component, a fibrous material are known. For example, Japanese Patent Application Publication No. 46-38443 (1971) discloses a friction material which has been prepared by mixing cellulose fibers, as a principal component, with fine solid particles a phenolic resin and placing the mixture in a mold and heating the mixture in the mold at an elevated temperature under pressure. Japanese Patent Application Publication No. 46- 21123 (1971) discloses a friction material which has been produced by incorporating a phenolic resin into a felt-like sheet in which an activated organic carbon is absorbed, and pressing the incorporated material in a mold at an elevated temperature under pressure. Japanese Patent Application Publication No. 48-24101 (1973) discloses a friction material which has been produced by pre-heating a mixture of wood pulp with a thermo-setting polymeric resin without pressing and pressing the preheated mixture in a mold at an elevated temperature under pressure. Also, Japanese Patent Application Laid-open No. 53-94279(1978) discloses a friction material which has been prepared in such a manner that a principal fibrous component comprising asbestos, wood pulp and inorganic fibers, for example, glass fibers or the like, are mixed with an inorganic filler and a metal oxide and, then, the mixture is converted into a sheet by a paper-making process.
Generally, the friction material usable for producing braking devices should exhibit satisfactory stable friction properties, heat-resistance and wear-resistance. When the friction material is used in a wet condition, it is necessary that the friction material have a satisfactory oil-holding property. Among the above-mentioned functions, the friction property and oil-holding property of the conventional friction materials are at a satisfactory level, respectively. Especially, the conventional friction materials containing a fibrous material, as a principal component, exhibit a superior friction property and oil-holding property under ordinary conditions. However, it is strongly desirable to enhance the friction property at an elevated temperature, for example, 250.degree. C. or more and also the wear-resistance of the conventional friction material.